The differing origins of gut dendritic cells — white blood cells that modulate immune responses — may explain how the intestinal immune system manages to destroy harmful pathogens while tolerating beneficial bacteria says an article by Sophie Laffont & Fiona Powrie in Nature journal out on Dec 10th 2009.
The immune system must protect the body from invading pathogens without mounting damaging responses to its own tissues. Dendritic cells, a rare population of white blood cells, have a crucial role in determining the nature of immune reactions and in fine-tuning the balance between tolerance (where the immune system ignores or tolerates an antigen) and the induction of inflammation to destroy pathogenic organisms.Dendritic cells are important when considering the role of immunonutrition and the application of probiotics as therapy, as this family of cells has the discriminatory ability to define friend from foe and direct overall adaptive as well as innate immune responses.
Pioneering work by Steinman and colleagues in the early 1970s identified a minor population of immune cells that they named dendritic cells on the basis of their stellate shape and membranous processes. These cells were shown to be potent stimulators of another population of white blood cells, T cells.
Dendritic cells are strategically placed within mucosal sites in the body, where they can detect infection and take up microbial antigens. On activation, these cells migrate to secondary lymphoid tissue, such as the lymph nodes, where they present the antigen to T cells. This activates the T cells, causing them to differentiate into effector cells that eradicate the pathogen. In the intestine, dendritic cells also promote regulatory T-cell responses that suppress immune reactions against beneficial commensal bacteria and food antigens, thereby preventing immune-related disease. Thus, intestinal dendritic cells are decision makers, ensuring selection of a T-cell response that is appropriate to the nature of the challenge to the immune system.
Keeping dendritic cells healthy and proportionate to response is one the mechanisms, favoured by the hygeine strategy to modulate local and systemic immune tolerance. The use of specific probiotics have demonstrated the ability to mature DC’s to promote the production of IL-10 and help maintain a position of immune unresponsiveness in the gastrointestinal and other tissues. These are sometimes called regulatory DC’s as they promote the creation of regulatory T cells and also assist in self tissue sampling reducing the risk of allergy and autoimmunity.,
References Laffont S, Powrie F.Immunology: Dendritic-cell genealogy. Nature. 2009 Dec 10;462(7274):732-3. No abstract available. PMID: 20010677 [PubMed – in process]  Smits HH, Engering A, van der Kleij D, de Jong EC, Schipper K, van Capel TM, Zaat BA, Yazdanbakhsh M, Wierenga EA, van Kooyk Y, Kapsenberg ML.Selective probiotic bacteria induce IL-10-producing regulatory T cells in vitro by modulating dendritic cell function through dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin. J Allergy Clin Immunol. 2005 Jun;115(6):1260-7. View Abstract  Latvala S, Pietila TE, Veckman V, Kekkonen RA, Tynkkynen S, Korpela R, Julkunen I. Potentially probiotic bacteria induce efficient maturation but differential cytokine production in human monocyte-derived dendritic cells. World J Gastroenterol. 2008 Sep 28;14(36):5570-83; View Full Article